1. Field of the Invention
This invention relates to a method and apparatus for producing foamed thermoplastic materials by homogeneously mixing thermoplastics with a foaming agent and other desired additives and continuously extruding the mixture.
2. Description of the Prior Art
There are various methods for producing foamed thermoplastic materials and extruders are now in wide use for this purpose. The producing of foamed thermoplastic materials using an extruder is practically advantageous in that, after a thermoplastic is homogeneously mixed with a foaming agent or other additive under pressure, the mixture is extruded under low pressure to continuously manufacture foamed theromplastic materials of desired shapes, e.g., sheets or plates.
In the above method of manufacture, however, importance has been attached to the fact that a foaming agent or other additive should homogeneously be mixed with the molten thermoplastic and the mixture extruded after the molten plastic composition containing the foaming agent is uniformly cooled sufficiently to be suitable for foaming.
As a result, there have been proposed various methods and apparatus for homogeneously mixing a thermoplastic with a foaming agent or the like and uniformly cooling the plastic composition containing the foaming agent. U.S. Pat. No. 3,751,377 discloses a method and apparatus wherein a static mixer is installed following an extruder having a screw in order to mix the molten plastic with a foaming agent and uniformly cool the mixture to a temperature fit for foaming. Although such an arrangement has succeeded in producing thick, large, low-density foamed materials to a certain extent, it has drawbacks in that the extrusion rate is reduced because the flow resistance of the plastics to the static mixer is strong, the static mixer is partially deformed or damaged particularly when it is cooled from the outside and the dispersion properties of the mixed foaming agent are insufficient.
Attempts have been made to remedy such shortcomings. As disclosed in U.S. Pat. No. 4,454,087, a rotary mixing cooler and a zigzag mixer as a kind of static mixer are simultaneously installed following the extruder. Although the zigzag mixer is prevented from deforming in this method because it is not cooled, there is still a desire to manufacture further low-density foamed materials and those in which the improved homogeneous dispersion of an additive is ensured.
On the other hand, U.S. Pat. No. 4,419,014 discloses a method for homogeneously mixing molten plastics, rubber and the like with other additives, wherein an extruder-mixer is equipped with a cavity transfer mixer directly coupled to the front end of the screw contained in the extruder.
The present inventors have conceived of the application of the cavity transfer mixer used in the above extruder-mixer to the manufacture of foamed thermoplastic materials and found that further improved foamed thermoplastic materials can be manufactured by introducing the cavity transfer mixer into the conventional extrusion process.